Modification of surface energy via direct laser ablative surface patterning

ABSTRACT

Surface energy of a substrate is changed without the need for any template, mask, or additional coating medium applied to the substrate. At least one beam of energy directly ablates a substrate surface to form a predefined topographical pattern at the surface. Each beam of energy has a width of approximately 25 micrometers and an energy of approximately 1-500 microJoules. Features in the topographical pattern have a width of approximately 1-500 micrometers and a height of approximately 1.4-100 micrometers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 12/894,279, now U.S. Pat. No. 8,987,632, filed Sep. 30, 2010, whichclaims benefit of priority to U.S. Provisional Patent Application61/250,190, with a filing date of Oct. 9, 2009. The contents of theforegoing applications are hereby incorporated by reference in theirentireties

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made in part by employees of the United StatesGovernment and may be manufactured and used by or for the Government ofthe United States of America for governmental purposes without thepayment of royalties thereon.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to methods for altering the surface energy of amaterial. More specifically, the invention is a method of modifying thesurface energy of a substrate material using laser ablation where themethod does not require the use of any template, mask, coating, orpost-ablation processing.

Description of the Related Art

Interfacial interactions are governed by the surface energy of thecontacting materials. The proclivity for favorable (adhesive or wetting)or unfavorable (abhesive or non-wetting) interactions will depend on therelative magnitude of these surface energies. As a result, the abilityto controllably alter a material's surface energy is of greatsignificance. Differences in the surface energy of materials can oftenbe observed via water contact angle values.

Surface chemistry and topography contribute to a material's surfaceenergy. Chemical functionalities present on a material's surface altersurface energy through intermolecular forces (e.g., dispersion forces,dipole-dipole interactions, polarizability, etc.). Topographies impactsurface energy by altering the contact line between two surfaces. Forliquid droplets on a solid surface, this is referred to as thethree-phase contact line. The continuity of the three-phase contact linedramatically influences the wetting behavior and surface energy of thesolid substrate. For a rough surface or one with a high density oftopographical features, the contact line can be continuous ordiscontinuous. If the contact line is continuous, the surface is said toexhibit wetting in the Wenzel state where the liquid has penetrated intothe interstices of the topographical features. This results in a largercontact area between the liquid and the surface than would be observedon a flat surface of the same dimensions and often leads to adhesionpromotion. For discontinuous contact lines, the liquid has notpenetrated the surface topography interstices resulting in a reducedcontact area relative to a flat surface and a Cassie-Baxter wettingstate. Discontinuous contact lines often correlate with surfaces thatexhibit abhesive interactions and superhydrophobicity, where watercontact angles exceed 150°.

Surface preparation for adhesion promotion utilizes methods to increasethe surface energy of a material either chemically or topographically.State-of-the art surface preparation for metal adherends typicallyinvolves grit-blasting followed by multiple chemical oxidativetreatments and subsequent application of a primer or coupling agent. Theuse of chemical surface preparation techniques for metallic substratesrequires large volumes of environmentally toxic materials. For thesurface preparation of reinforced composite materials grit-blasting,manual abrasion, and peel ply treatments are commonly employed. All ofthese surface preparation techniques are not ideal in terms ofadhesively bonded structures because of variations in their application(i.e., there can be dissimilarities across surfaces because of differentoperators, operator error, or other inconsistencies inherent with thesetechniques). Similarly, these techniques can alter the bonding interfacedue to debris and introduction of surface curvature. Thus, anenvironmentally benign, rapid, scalable, precise, and highlyreproducible surface preparation technique for the purposes of adhesionpromotion would mitigate many of these shortcomings and be of greatutility.

Surface preparation for abhesion promotion requires a reduction in thesurface energy of a material. Once again, this can be achieved bothchemically and topographically. For a smooth surface, water contactangle values >120° cannot be achieved solely through surface chemicalmodification. For the generation of superhydrophobic surfaces, which areakin to low surface energy materials, topographical modification of thematerial is required. Superhydrophic surfaces are known to mitigateparticulate adhesion, which not only changes the appearance of anexposed surface, but also can impair or reduce the efficacy of theimpacted structure. Exterior building wall fouling as a result ofparticulate accumulation often results in acceleration of degradationdue to the introduction of organic matter and a viable matrix for moldand fungal growth. Similarly, solar cell efficiency rapidly diminishesas a result of surface contamination by particulate adhesion. Frictionalwear also increases considerably due to the presence of particulatematter. Therefore, identification of a method to reduce the propensityfor particulate adhesion by lowering surface energy via topographicalmodification would be useful to a broad range of materials applications.

The current state-of-the-art surface treatment for aluminum metalbonding for most applications is phosphoric acid anodization, chromicacid anodization, or chromic acid etching. The preferred surfacetreatment method for both production and repair of titanium, stainlesssteel, and nickel substrates is a wet chemistry process called sol-gel.Although great progress has been made over the past few decades inimproving the performance and durability of bonded metal structures,there remains much room for improvement. Furthermore, one of thegreatest challenges facing the metal bonding industry today are thechanging safety and environmental regulations that control the use ofchemicals used to process bonded metal structures. There is a great needto minimize or eliminate the use of toxic chemicals and volatile organicsolvents.

The current state-of-the-art for preparing composite surfaces forbonding uses abrasive techniques such as grit-blasting, surfaceroughening (manual abrasion), and peel plies. These methods lackprecision and reproducibility thereby making quality control difficult.Surface preparation methods for composites are currently processcontrolled and no viable methods exist to assess whether a surface isadequately prepared. Also, the reliability of peel ply methods needsadditional improvement from a reproducibility and contaminationviewpoint. While grit-blasting of composite surfaces is widely used, theunderstanding of the effects of microcracking and grit embedment stillneed to be understood within the context of the durability of the bondas it ages. From an environmental and health perspective, thecontainment of the grit blast media and exposure of workers to grit dustare also issues.

A technique for generating patterned surface topographies that includeslaser ablation was reported by Jin et al. in “Super-Hydrophobic PDMSSurface with Ultra-Low Adhesive Force,” Macromolecular RapidCommunications, 26, 1805-1809 (2005). However, the technique describedtherein requires laser ablation of a support substrate utilizing a maskor template to impart the desired surface pattern. The ablation processalso results in deposition of ablated debris on the treated surface. Asa result, this ablated material forms topographical features on thenanometer scale that render the material superhydrophobic. A furtherexample of laser ablation patterning was reported by Schulz et al. in“Ultra Hydrophobic Wetting Behavior of Amorphous Carbon Film,” Surfaceand Coatings Technology, 200, 1123-1126 (2005). Surface energy reductiondescribed therein requires the use of both a laser and an arc plasmagenerating device. Additionally, the surfaces were renderedsuperhydrophobic only after the addition of an undisclosed hydrophobicfilm.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asimple and environmentally-safe method of modifying the surface energyof materials for the purposes of adhesion or abhesion promotion.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a method of changing thesurface energy of a substrate without the need of any template, mask, oradditional coating medium is provided. A completely uncovered surface ofa substrate is directly ablated using at least one beam of energy toform a predefined topographical pattern at the surface. Each beam ofenergy has a width of approximately 25 micrometers and an energy ofapproximately 1-500 microJoules. Features in the topographical patternhave a width of approximately 1-500 micrometers and a height ofapproximately 1.4-100 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a laser ablation set-up in accordance withthe present invention;

FIG. 2 is a perspective view of a substrate material illustrating a0°/90° laser-ablated crosshatch pattern in accordance with an embodimentof the present invention;

FIG. 3 is a schematic view of a substrate and adhesive in accordancewith the present invention;

FIG. 4 is a schematic view of a substrate material illustrating a 0°/45°laser-ablated crosshatch pattern in accordance with an embodiment of thepresent invention; and

FIG. 5 depicts characterization results for surface treatment of variouscopoly(imide siloxane)s as described in Example 10.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to a laser ablative method tocontrollably alter the surface energy of a material via directtopographical modification using specific geometric patterns. The word“direct” as used herein is used to describe modification of a substratewithout the need or use of any template, mask, or additional coatingmedium being applied to the surface of the substrate. More specifically,the present invention precisely controls laser ablation parametersincluding but not limited to beam size, laser power, laser frequency,scan speed, and number of pattern iterations. This enables thegeneration of topographical features that influence the surface energyof the material, which in turn control adhesive and abhesive properties.Since laser ablation is a material dependent process, laser ablationparameters requisite for introduction of the desired material property(surface energy) will vary depending on the substrate utilized. Thislaser ablative method is useful in a variety of applications includingbut not limited to replacing of Pasa-Jell or other chemical treatmentsfor titanium bonding, replacing of peel-ply treatments for carbon-fiberreinforced composite bonding, generating coatings or surfaces foranti-icing, de-icing, and anti-insect adhesion on aerospace vehicles,generating low friction coatings or surfaces, generating light-absorbingcoatings or surfaces, and generating self-cleaning coatings or surfaces.

This laser ablative method can be performed on metals, metal alloys,ceramics, polymers, and fiber reinforced metal, polymer, or ceramiccomposites or combinations thereof. Preferred materials include titaniumand aluminum substrates, carbon fiber reinforced composite materials,and polymeric materials. The method is applicable to any polymericmaterial. Preferred materials include but are not limited to commodityor engineering plastics including but not limited to polycarbonate,polyacrylate, polyacrylonitrile, polyester, polyamide, polystyrene(including high impact strength polystyrene), polyurethane, polyurea,polyurethaneurea, epoxy resins, poly(acrylonitrile butadiene styrene),polyimide, polyarylate, poly(arylene ether), polyethylene,polypropylene, polyphenylene, polyphenylene sulfide, poly(vinyl ester),polyether ether ketone, polyvinyl chloride, poly(vinyl alcohol),bismaleimide polymer, polyanhydride, liquid crystalline polymer,cellulose polymer, fluorinated polymers, or any combination thereof. Themethod is also applicable to copolymers of the aforementioned polymericmaterials and preferred copolymers are copoly(imide siloxane)s,copoly(imide butadiene)s, copoly(imide butadiene acrylonitrile)s. Manyof these polymers are available from multiple, well-known commercialsuppliers.

The materials can be in a variety of forms such as foams, films,coatings, fibers, adhesives, molded or machined parts consisting of asingle or multiple material compositions. The method can be used tomodify surfaces during the original manufacture or to modify surfaces inthe field as part of a remanufacturing or repair process.

A simplified view of the approach used in the present invention isillustrated in FIG. 1. In this method, a substrate 10 to belaser-ablation-patterned is positioned in reference to the irradiationsource such as a laser 12 that transmits a beam of energy 14 towardssubstrate 10. For the purposes of demonstrating the invention describedherein, a frequency tripled (λ=355 nm, 7 W) Nd:YAG laser was utilized ina pulsed mode. However, any source of laser irradiation of sufficientenergy to supersede the ablation threshold of the substrate material canbe envisioned including but not limited to a CO₂ laser source, anexcimer laser source, a high power diode laser source, a Ti:Sapphirelaser source, and different frequencies of a Nd:YAG laser source or acombination thereof. Although the Nd:YAG laser was operated in a pulsedmode, continuous wave laser irradiation can also be used to controllablyalter the surface energy of an exposed material through topographicalmodification. For the purposes of pattern transcription, agalvanometrically driven beam scanner (not shown) was utilized toprecisely and programmatically control the position and speed of thelaser beam. Other embodiments for pattern transcription include but arenot limited to programmatically-controlled movement of the substrate,manually-controlled movement of the laser, and manually-controlledmovement of the substrate or any combination thereof.

In general, a predefined pattern is transcribed on the substrate whilecontrolling the beam size, laser power, laser frequency, scan speed, andnumber of pattern iterations. Preferred ablation patterns include 0°/90°crosshatch patterns, 0°/45° crosshatch patterns, linear array patterns,and orthogonal rotating linear arrays. Other patterns that can beenvisioned include but are not limited to circular arrays, fractalgeometries, triangular arrays, concentric patterns, diamond arrays,curved linear arrays, curved crosshatch patterns, and crosshatchpatterns with other relative angles. FIG. 2 is an example of a 0°/90°crosshatch pattern in which substrate 10 has its surface ablated toyield rectangular pillars 16 of width “W” and height “H”. Pillars 16 areseparated by gaps 18 formed during laser ablation. Similarly, FIG. 4schematically shows an example of a 0°/45° crosshatch patterntranscribed on a substrate material.

A variety of substrate materials were laser ablated in accordance withthe method of the present invention. These examples are presented laterherein. In some examples, laser ablation patterning resulted in theformation of adhesion promoting surfaces (i.e., surfaces with increasedsurface energy). Laser ablation patterning for adhesion promotionrequires that the laser parameters be adjusted to transcribetopographical features within geometric constraints. The maximal spacingbetween ablation lines should be no greater than about 300 μm.Similarly, an ablation depth of about 10 μm or greater must be achieved.These surfaces will exhibit a water contact angle lower than theun-ablated surface and typically less than about 50° corresponding tosurface energy values greater than about 49 milliJoules per metersquared (mJ/m²) as calculated by water contact angle measurement.

In other examples, the use of laser ablation patterning resulted in lowsurface energy materials with abhesive properties. Laser ablationpatterning for the lowering of surface energy and abhesion promotionalso requires that the laser parameters be adjusted to transcribetopographical features within geometric constraints. The topographicalfeatures on surfaces engineered for abhesive applications should possessa minimal ablation depth of about 1.4 μm. Ablation depth maxima willdepend on the spacing of topographical features, the dimensions of thefeatures, and the rigidity of the ablated material. For highly rigidmaterials possessing a flexural modulus of at least about 1.5 GPa, therewill be no ablation depth maximum. For materials with low rigiditypossessing a modulus equal to or less than about 1.5 GPa, an ablationdepth maximum would be the maximum depth that would enable maintenanceof the orientation of the topographical features (i.e., the featureswould not persist in an orientation different than the one in which theywere generated). The spacing and dimensions of the topographicalfeatures will be dependent on the ablation depth. In some embodiments itis preferred that the surface topographical features not be spaced muchfurther apart than the magnitude of the feature dimension multiplied bythe ablation depth. Typical properties exhibited by abhesion promotinglaser ablation patterned surfaces include increased water contact anglevalues relative to the un-ablated surface and typically greater thanabout 90° resulting in surface energy values less than about 18.2 mJ/m².Similarly, for the purposes of generating superhydrophobic surfaces, theresultant geometry should generate a highly discontinuous three-phasecontact line. These surfaces may exhibit contact angle hysteresis values(the difference in advancing and receding contact angles) less thanabout 30° and sliding angles (the angle at which the substrate is tiltedto induce rolling of the incident water droplet) less than about 15°.

In some embodiments it may be desirable to have more than one lasertreatment step. For example, an Nd:YAG laser may be used to produce apatterned surface. A secondary laser treatment, with the same ordifferent laser, may be used to further treat the surface to create thedesired surface patterns and properties. In other embodiments, it may bedesirable to use multiple lasers to produce a surface pattern withhierarchical dimensional features (i.e., smaller features on top oflarger features) as a means to generate desired surface properties.

Articles incorporating materials with surfaces modified by the laserablation methods described herein include articles such as, but notlimited to, titanium and aluminum bonding specimens enabling thereplacement of Pasa-Jell or other chemical treatments, carbon-fiberreinforced composite bonding specimens enabling the replacement ofpeel-ply treatments, and articles requiring anti-icing, de-icing, andanti-insect adhesion, low friction surfaces, light absorbing and/orscattering surfaces, and self-cleaning surfaces. These articles couldalso be a component of a larger assembly including but not limited toterrestrial and aerospace vehicles, solar panel assemblies, black bodydetectors, microelectronic components and the fabrication processthereof, dust-resistant articles, and moisture uptake resistantmaterials.

In summary, the present invention discloses a method to controllablymodify the surface energy of a material by generating topographicalpatterns via direct laser ablation. Depending on the laser parametersutilized, hydrophilic materials can be transformed into hydrophobic andsuperhydrophobic surfaces, or can be modified to exhibit increasedhydrophilicity. Contrarily, hydrophobic materials can be modified toexhibit surface properties ranging from hydrophilic to superhydrophobic.The laser ablation patterning method is rapid, scalable, environmentallybenign, precise, and can be performed directly on a wide variety ofmaterials. The resultant articles of manufacture have surfaces that canbe utilized for adhesive bonding, self-cleaning, particle adhesionmitigation, low friction surfaces, anti-icing, de-icing, and anti-insectadhesion applications, and light scattering devices such as black bodyinstruments among other embodiments.

Prior to describing specific examples fabricated using the method of thepresent invention, the general parameter constraints associated withsome embodiments of the present invention will be presented. In oneembodiment of the present invention, ablation is performed using one (ormore) beams of energy having a beam width of approximately 25micrometers and an energy of approximately 1-500 microJoules. Theresulting features can have a width of approximately 1-500 micrometersand a height of approximately 1.4-100 micrometers. In another embodimentof the present invention, ablation is performed using one (or more)beams of energy having a beam width of approximately 25 micrometers andan energy of approximately 1-200 microJoules. The resulting features canhave a width of approximately 10-250 micrometers and a height ofapproximately 1.4-50 micrometers. In still another embodiment of thepresent invention, ablation is performed using one (or more) beams ofenergy having a beam width of approximately 25 micrometers and an energyof approximately 3-175 microJoules. The resulting features can have awidth of approximately 15-100 micrometers and a height of approximately10-30 micrometers.

EXAMPLES

Having generally described the invention, a more complete understandingthereof may be obtained by reference to the following examples that areprovided for purposes of illustration only and do not limit theinvention.

Example 1. Generation of Hydrophilic Titanium Ti-6Al-4V Surfaces

The surface of titanium alloy (Ti-6Al-4V) lap shear specimens wasmodified by either grit-blasting, laser ablation patterning, orgrit-blasting followed by laser ablation patterning. For laser ablationpatterning an Nd:YAG laser (•=355 nm) with a beam size of 25 •m,operating at 6.3 W and 30 kHz (resulting in a pulse energy of 210 •J)with a scan speed of 25.4 cm/s was used to transcribe a 0°/90°crosshatch pattern using a single transcription step. The pattern wastranscribed in the surface by first etching parallel lines in onedirection. Next, parallel lines were drawn over the same sample spacewith a perpendicular orientation to the first series of lines at thesame line spacing. The line spacing between features for each surfaceablation treatment is indicated in Table 1. This ablation patterncreated a square pillar array with a pillar width of 220 •m and anaverage feature height of 20 •m. The surface energy was determined usingwater contact angle values (Table 1). The treated titanium surfaces weresubsequently coated with a primer or coupling agent consisting of imideoligomers with phenylethynyl pendant functionalities end-capped withtrimethoxysilane groups following a procedure described by Park et al.in “Polyimide-Silica Hybrids Using Novel Phenylethynyl Imide Silanes asCoupling Agents for Surface-Treated Alloy,” International Journal ofAdhesion and Adhesives, 20, 457-465 (2000). These samples were thenbonded as described in Park et al. and the apparent shear strength wasdetermined according to ASTM D1002-05 (Table 1).

As seen in Table 1, laser ablation patterning resulted in a dramaticdecrease in water contact angle, a dramatic increase in surface energy,and comparable, if not superior, apparent shear strength values relativeto samples that were only grit-blasted. Thus, it is clear that thepresent invention affords a rapid, scalable, highly precise,reproducible method for surface energy modification for titanium.

TABLE 1 Characterization results for surface treatment of titaniumTi—6Al—4V lap shear specimens as described in Example 1. Water ApparentLine Contact Surface Shear Surface Spacing Angle Energy StrengthTreatment (μm) (°) (mJ/m²) (MPa) Pristine N/A 74 29.6 N/A Surface Grit-N/A 91 17.6 29.3 ± 0.9 Blasted Laser 102 2 72.7 30.2 ± 0.9 AblationPatterned Laser 254 2 72.7 18.4 ± 0.6 Ablation Patterned Laser 406 272.7 15.5 ± 1.5 Ablation Patterned Grit- 254 2 72.7 29.6 ± 0.6 Blastedand Laser Ablation Patterned

Example 2. Generation of a Hydrophilic Aluminum 6061 Surface

An aluminum coupon (Al 6061) was exposed to the same laser ablationconditions as described in Example 1 except the laser scan rate was 12.7cm/s, and the line spacing for the crosshatch pattern was 25 •m. Thisablation pattern created a square pillar array with pillar width of 22•m on the treated surface with an average feature height of 18 •m. Thesurface energy was determined using water contact angle values. Apristine Al 6061 surface exhibited a water contact angle value of 98°corresponding to a surface energy of 13.5 mJ/m². The laser ablationpatterned surface exhibited a water contact angle of 2.0° correspondingto a surface energy of 72.7 mJ/m².

Example 3. Generation of a Hydrophobic Aluminum 6061 Surface

An aluminum coupon (Al 6061) was modified by laser ablation patterningwith an Nd:YAG laser (•=355 nm) with a beam size of 25 •m, operating at6.3 W and 80 kHz (resulting in a pulse energy of 78.8 •J) with a scanspeed of 25.4 cm/s. A 0°/90° crosshatch pattern with a line spacing of25 •m was transcribed four times. This ablation pattern created a squarepillar array with pillar widths of 22 •m and an average feature heightof 15 •m. The surface energy was determined using water contact anglevalues. A pristine Al 6061 surface exhibited a water contact angle of102° corresponding to a surface energy of 11.4 mJ/m². The laser ablationpatterned surface exhibited a water contact angle of 108° correspondingto a surface energy of 8.7 mJ/m².

Example 4. Generation of Hydrophilic Carbon Fiber Reinforced CompositeSurfaces

Carbon fiber reinforced composite specimens (16 plies of unidirectionalTorayca P2302-19 prepreg, a T800H/3900-2 carbon fiber-toughened epoxysystem) were modified by grit-blasting, wet-peel ply (material), drypeel ply (material), and laser ablation patterning. Two different laserablation patterns were utilized. Pattern A was a 0°/90° crosshatch whilePattern B was created to replicate that of a peel ply treated surfaceand consisted of an orthogonal rotation of linear arrays with linewidths of 25 •m and linear arrays consisting of 15 lines with a linespacing of 200 •m. Each linear array, 0.3 cm in length and width, wasoriented at 90° to the surrounding linear arrays. For laser ablationpatterning, an Nd:YAG laser (•=355 nm) with a beam size of 25 •m, a linespacing of 25 •m, and a scan speed of 25.4 cm/s was used. These patternswere transcribed into the surface using a single transcription step.Laser power and frequency were varied as indicated in Table 2. Thesurface energy was determined using water contact angle values (Table2).

The treated composite surfaces were subsequently bonded with AF-555Madhesive (available commercially from the 3M Company) according to themanufacturer's specifications and the apparent shear strength wasdetermined using a slight modification of ASTM D3165-00 regarding howthe bonded test specimens were gripped (Table 2).

As seen in Table 2, laser ablation patterning dramatically reduced thewater contact angle and increased the surface energy. Also, the apparentshear strength of the laser ablation patterned specimens is comparable,if not superior, to values obtained with other surface preparationtechniques. This approach affords a rapid, scalable, highly precise,reproducible method for surface treatment of carbon fiber reinforcedcomposites and eliminates contamination sources for bonded areascompared to peel-ply surface treatments.

TABLE 2 Characterization results for surface treatment of carbon fiberreinforced composite specimens as described in Example 4. Laser Power(W)/ Water Apparent Frequency Contact Surface Shear Surface (kHz)/PulseAngle Energy Strength Treatment Energy (μJ) (°) (mJ/m²) (MPa) PristineN/A 79 25.8 23.9 ± 1.2 Grit- N/A 86 20.8 25.1 ± 1.0 Blasting Wet PeelN/A 76 28.1 25.5 ± 0.7 Ply Dry Peel N/A 83 22.9 26.7 ± 1.7 Ply Pattern A5.6/40/140 2 72.7 27.4 ± 1.3 Pattern A 5.6/60/93.3 14 70.6 27.6 ± 0.9Pattern A 6.3/30^(a)/210 2 72.7 26.4 ± 0.6 Pattern B 5.6/40/140 26 65.626.7 ± 0.7 ^(a)The crosshatch pattern line spacing was 50 μm.

Example 5. Generation of a Hydrophobic Carbon Fiber Reinforced CompositeSurface

Carbon fiber reinforced composite specimens (16 plies of unidirectionalTorayca P2302-19 prepreg, a T800H/3900-2 carbon fiber-toughened epoxysystem) were modified via laser ablation patterning using Patterns A andB from Example 4 with the same laser ablation parameters except thelaser power was 4.9 W operating at 60 kHz (resulting in a pulse energyof 81.7 •J).

The surface energy was determined using water contact angle values.Pristine carbon fiber reinforced composite surfaces exhibited a watercontact angle of 79° corresponding to a surface energy of 25.8 mJ/m².The laser ablation patterned surfaces exhibited water contact angles of100° and 101° corresponding to surface energies of 12.4 mJ/m² and 11.9mJ/m² for Patterns A and B, respectively.

Example 6. Generation of a Hydrophilic Polymer Surface

The surface of a polyphenylene material (Primospire® PR250, SolvayAdvanced Polymers®) was modified by laser ablation patterning similar toExample 3 except the laser was operated at 5.6 W and 80 kHz (resultingin a pulse energy of 70 •J). This ablation pattern created a squarepillar array with pillar widths of 15 •m on the treated surface and anaverage feature height of 10 •m.

The surface energy was determined using water contact angle values.Pristine Primospire® PR250 exhibited a water contact angle of 87°corresponding to a surface energy of 20.1 mJ/m². The laser ablationpatterned surface exhibited a water contact angle of 46° correspondingto a surface energy of 52.2 mJ/m².

Example 7. Generation of a Hydrophobic Polymer Surfaces

Kapton® HN polyimide film specimens (available commercially from DuPontde Nemours Co.) were modified by laser ablation patterning in a mannersimilar to Example 3 except the laser was operated at power settings andtranscription steps as indicated in Table 3. This ablation patterncreated a square pillar array with pillar widths of 25 •m and averagefeature heights as indicated in Table 3. The surface energy wasdetermined using water contact angle values and is indicated in Table 3.

TABLE 3 Characterization results for surface treatment of Kapton ® HNspecimens as described in Example 7. Water Laser Power Number of ContactSurface Surface (W)/Pulse Pattern Angle Energy Feature Energy (μJ)Transcriptions (°) (mJ/m²) Height (μm) Pristine Surface 81 24.3 N/A4.9/61.3 1 83 22.9 1.4 4.9/61.3 2 82 23.6 2.2 4.9/61.3 4 92 16.9 4.15.1/63.8 1 83 22.9 3.5 5.1/63.8 2 91 17.6 4.4 5.1/63.8 4 98 13.5 12.55.3/66.3 1 85 21.5 5.9 5.3/66.3 2 89 18.8 6.5 5.3/66.3 4 108 8.7 8.5

Example 8. Generation of Hydrophobic Polymer Surfaces from CommercialSource Materials

Film specimens from several commercial sources including: APEC® 2097(Bayer Materials Science, LLC), Teflon® (DuPont de Nemours Co.), and acrystalline PEEK film (Ajedium Films Croup, LLC.) were modified by laserablation patterning in a manner similar to Example 3 except the laserwas operated at power settings as indicated in Table 4. The surfaceenergy was determined using water contact angle values and is indicatedin Table 4.

TABLE 4 Characterization results for surface treatment of commercialmaterials as described in example 8. Water Laser Power Contact Surface(W)/Pulse Angle Energy Material Energy (μJ) (°) (mJ/m²) APEC ® 2097Pristine 93 16.3 Surface APEC ® 2097 5.3/66.3 95 15.2 APEC ® 20975.6/70   102 11.4 Teflon ® Pristine 109 8.3 Surface Teflon ® 5.3/66.3109 8.3 Teflon ® 5.6/70   120 4.5 Teflon ® 6.3/78.8 121 4.3 PEEKPristine 85 21.5 Surface PEEK 5.3/66.3 96 14.6 PEEK 5.6/70   88 19.5

Example 9. Generation of a Hydrophobic Copoly(Imide Siloxane) Surface

Copoly(imide siloxane) specimens were generated from the condensationreaction of an aromatic dianhydride(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 6FDA) with amixture of an aromatic diamine (4,4′-oxydianiline, 4,4′-ODA) and anamine-terminated polydimethyl siloxane (DMS-A21, Gelest, 10 wt. %).Reactions were carried out under nitrogen using a 1:1 ratio ofdianhydride and diamine (20 wt. % solids) in a 4:1 mixture ofN-methylpyrrolidinone (NMP) and tetrahydrofuran (THF). The diamine wasdissolved in NMP, to which a THF solution of DMS-A21 was added, followedby the dianhydride and additional NMP. The reaction mixture wasmechanically stirred overnight. Films were cast on a Teflon® coatedsurface or polished stainless steel using a doctor's blade and placed ina forced air drying chamber until “tack-free.” Films were then thermallyimidized under nitrogen using a cure cycle with stages at 150, 175, 200,and 250° C.

Film specimens were modified by laser ablation patterning in a mannersimilar to Example 3 except the laser was operated at power andfrequency settings as indicated in Table 5. This ablation patterncreated a square pillar array with pillar widths and average featureheights indicated in Table 5. The surface energy was determined usingwater contact angle values and is indicated in Table 5.

TABLE 5 Characterization results for surface treatment of copoly(imidesiloxane), 6FDA:4,4′-ODA: DMS-A21 (5 wt. %) as described in Example 9.Surface Laser Power Number of Water Surface Feature (W)/Pulse PatternContact Energy Height Energy (μJ) Transcriptions Angle (°) (mJ/m²) ( 

m) Pristine Surface 112 7.1 N/A 4.9/61.3 1 132 2.0 1.5 4.9/61.3 2 1341.7 1.8 4.9/61.3 4 139 1.1 1.9 5.1/63.8 1 139 1.1 3.6 5.1/63.8 2 143 0.76.0 5.1/63.8 4 149 0.4 9.1 5.3/66.3 1 142 0.8 7.2 5.3/66.3 2 151 0.310.5

Example 10. Generation of a Superhydrophobic Copoly(Imide Siloxane)Surface

Copoly(imide siloxane) specimens were generated from the samecondensation reaction described in Example 9 except the aromaticdianhydride used was either 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 6FDA, or 4,4′-oxydiphthalic anhydride,ODPA and the aromatic diamine was: 3,4′-oxydianiline, 3,4′-ODA;4,4′-oxydianiline, 4,4′-ODA; 1,3-bis(3-aminophenoxy) benzene, 1,3-APB;or 2,2-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane, 4-BDAF. Specificmonomer combinations are indicated in Table 6. The DMS-A21 weightpercent was also varied as indicated in Table 6. Film specimens weremodified by laser ablation patterning in a manner similar to Example 3except the laser was operated at 5.25 W (resulting in a pulse energy of65.6 •J).

The surface energy was determined using water contact angle values andis indicated in Table 6. Advancing and receding contact anglemeasurements and sliding angle measurements were made by tilting axiswater contact angle measurements.

TABLE 6 Characterization results for surface treatment of variouscopoly(imide siloxane)s as described in Example 10. Laser AblationPatterned Surface Pristine Surface Surface DMS-A21 Surface FeatureSurface Material Content θ_(adv) Energy Sliding Height θ_(adv) EnergySliding Composition (wt. %) (θ_(reg)), ° (mJ/m²) Angle (μm) (θ_(reg)), °(mJ/m²) Angle 6FDA: 4,4′-ODA 5 112 7.1 43 11.3 167 0.01 10 (88) (140)6FDA: 4,4′-ODA 10 102 11.4 31 16.5 163 0.03 10 (80) (154) 6FDA: 4,4′-ODA20 113 6.8 >60 13.0 171 0.002 2 (95) (164) 6FDA: 3,4′-ODA 10 115 6.1 2718.3 170 0.004 2 (90) (159)

Example 11. Generation of a Superhydrophobic Copoly(Imide Butadiene)Surface

Copoly(imide butadiene) specimens were generated from the condensationreaction of an aromatic dianhydride(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 6FDA) with amixture of an aromatic diamine (1,3-bis(3-aminophenoxy) benzene,1,3-APB) and an amine-terminated polybutadiene (generated by thereaction of hydroxyl-terminated polybutadiene with p-nitrobenzoylchloride and the subsequent reduction of the nitro functionalities usingtin chloride dihydrate). Reactions were carried out under nitrogen usinga 1:1 ratio of dianhydride and diamine (20 wt. % solids) in a 4:1mixture of N-methylpyrrolidinone (NMP) and toluene. The diamine wasdissolved in NMP, to which a toluene solution of amine-terminatedpolybutadiene was added, followed by the dianhydride and additional NMP.The reaction mixture was mechanically stirred overnight. Films were caston glass plates using a doctor's blade and placed in a forced air dryingchamber until “tack-free.” Films were then thermally imidized undernitrogen using a cure cycle with stages at 150, 175, 200, and 250° C.Film specimens were modified by laser ablation patterning in a mannersimilar to Example 10.

The surface energy was determined using water contact angle values.Pristine copoly(imide butadiene) exhibited a water contact angle of 83°corresponding to a surface energy of 22.9 mJ/m². The laser ablationpatterned surface exhibited a water contact angle of 175° correspondingto a surface energy of 0.0003 mJ/m². The surface also exhibited acontact angle hysteresis of 9.5° and a sliding angle of 3°.

Example 12. Generation of a Superhydrophobic Copoly(Imide ButadieneAcrylonitrile) Surface

Copoly(imide butadiene acrylonitrile) specimens were generated from thecondensation reaction of an aromatic dianhydride(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 6FDA) with amixture of an aromatic diamine (3,4′-oxydianiline, 3,4′-ODA) and anamine-terminated copoly(butadiene acrylonitrile) (Polysciences Inc.,Product No. 09753). Reactions were carried out under nitrogen using a1:1 ratio of dianhydride and diamine (20 wt. % solids) in a 4:1 mixtureof N-methylpyrrolidinone (NMP) and toluene. The diamine was dissolved inNMP, to which a toluene solution of amine-terminated copoly(butadieneacrylonitrile) was added, followed by the dianhydride and additionalNMP. The reaction mixture was mechanically stirred overnight. Films werecast on glass plates using a doctor's blade and placed in a forced airdrying chamber until “tack-free.” Films were then thermally imidizedunder nitrogen using a cure cycle with stages at 150, 175, 200, and 250°C. Film specimens were modified by laser ablation patterning in a mannersimilar to Example 10.

The surface energy was determined using water contact angle values.Pristine copoly(imide butadiene acrylonitrile) exhibited a water contactangle of 86° corresponding to a surface energy of 20.8 mJ/m². The laserablation patterned surface exhibited a water contact angle of 173°corresponding to a surface energy of 0.001 mJ/m². The surface alsoexhibited a contact angle hysteresis of 20.8° and a sliding angle of 7°.

The examples provided herein serve to demonstrate the nature of thisinvention, which is a method to controllably modify the surface energyof a variety of materials by the generation of topographical patterns ofspecific dimensional sizes and geometric shapes via direct laserablation. The examples demonstrate that depending on the laserparameters utilized, hydrophilic materials can be rendered morehydrophilic, hydrophobic or superhydrophobic, and hydrophobic materialscan be modified to exhibit hydrophilic, more hydrophobic orsuperhydrophobic surface properties. The laser ablation patterningmethod is rapid, scalable, environmentally benign, precise, and can beperformed on a wide variety of materials. The resultant surfaces can beutilized for adhesive bonding, self-cleaning, particle adhesionmitigation, low friction surfaces, and anti-icing surfaces to name afew.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

The invention claimed is:
 1. A method of changing the surface energy ofa substrate to promote abhesion, the method comprising: directlyablating a completely uncovered surface of the substrate using at leastone laser beam of energy to form a predefined topographical pattern atthe surface, the at least one laser beam having a width of 25micrometers and an energy of 1-200 microJoules; wherein directlyablating the completely uncovered surface of the substrate includesmoving the at least one laser beam along a 0°/90° crosshatch patternsuch that the predefined topographical pattern includes an array ofsquare pillars having a width of 10-250 micrometers and a height of1.4-50 micrometers; wherein the directly ablating step renders thesubstrate hydrophobic without applying any template, mask, or additionalcoating medium to the substrate; and wherein the substrate is selectedfrom the group consisting of metals, metal alloys, ceramics, polymers,fiber-reinforced composites thereof, and combinations thereof.
 2. Themethod of claim 1, wherein the predefined topographical pattern has atleast one property selected from the group consisting of anti-icing,de-icing, anti-insect adhesion, low friction, light modifying, andself-cleaning properties.
 3. The method of claim 1, wherein: thepredefined topographical pattern is characterized by a water contactangle at or above 90° and a sliding angle of less than 15°, the slidingangle being an angle at which the substrate is tilted to induce rollingof an incident water droplet.
 4. The method of claim 1, wherein: thestep of directly ablating a completely uncovered surface of thesubstrate using at least one laser beam of energy to form a predefinedtopographical pattern at the surface comprises a multi-step processfurther including moving the at least one laser beam along secondpattern comprising an orthogonal rotation of linear arrays.
 5. A methodof changing the surface energy of a substrate to promote abhesion, themethod comprising: directly ablating a completely uncovered surface of asubstrate using at least one laser beam of energy to form a predefinedtopographical pattern at the surface, the at least one laser beam ofenergy having a width of 25 micrometers and an energy of 3-175microJoules; wherein directly ablating the uncovered surface of thesubstrate includes moving the at least one laser beam along a 0°/90°crosshatch pattern such that the predefined topographical patternincludes an array of square pillars having a width of 15-100 micrometersand a height of 10-30 micrometers; and wherein the directly ablatingstep comprises controlling laser-ablation parameters selected from thegroup consisting of beam size, laser power, laser frequency, scan speed,number of pattern iterations, and combinations thereof, with thelaser-ablation parameters controlled so that the laser ablation formsthe predefined topographical pattern and thereby renders the substratehydrophobic without applying any template, mask, or additional coatingmedium to the substrate.
 6. The method of claim 5, having the pattern,wherein the predefined topographical pattern has at least one propertyselected from the group consisting of anti-icing, de-icing, anti-insectadhesion, low friction, light modifying, and self-cleaning properties.7. The method of claim 6, wherein the substrate comprises one or morecomponents selected from the group consisting of metals, metal alloys,ceramics, polymers, fiber-reinforced composites thereof, and anycombinations thereof.
 8. The method of claim 5, wherein: the substrateis composed of a metal alloy; and the metal alloy is at least one ofTi-6Al-4V and Al
 6061. 9. The method of claim 5, wherein: the substrateis composed of a polymer; and the polymer is selected from a groupconsisting of polyimide, copoly(imidesiloxane), copoly(imide butadiene),copoly(imidebutadiene acrylonitrile), polycarbonate, poly(aryleneether), fluoropolymer, epoxy resins, and polyphenylene.
 10. The methodof claim 5, wherein the substrate is composed of a fiber-reinforcedcomposite of T800H/3900-2.
 11. The method of claim 5, wherein: thepredefined topographical pattern is characterized by a water contactangle at or above 90° and a sliding angle of less than 15°, the slidingangle being an angle at which the substrate is tilted to induce rollingof an incident water droplet.
 12. A method of changing the surfaceenergy of a substrate, the method comprising: directly ablating anuncovered surface of a substrate using at least one laser beam to form apredefined topographical pattern, including: etching a first series ofparallel lines at a predefined line spacing; moving the at least onelaser beam along the first series of parallel lines such that thepredefined topographical pattern includes a linear array; wherein theuncovered surface of the substrate is rendered hydrophobic without anytemplate, mask, or additional coating medium applied to the substrate;and wherein the predefined topographical pattern is characterized by awater contact angle at or above 90° and a sliding angle of less than15°, the sliding angle being an angle at which the substrate is tiltedto induce rolling of an incident water droplet.
 13. The method of claim12, further comprising: etching another set of parallel lines with a 90°orientation to the first series of parallel lines with the predefinedline spacing to create a 0°/90° crosshatch pattern; and moving the atleast one laser beam along the 0°/90° crosshatch pattern such that thepredefined topographical pattern includes an array of square pillarshaving a width of 1-100 micrometers and a height of 1.4-100 micrometers.14. The method of claim 12, further comprising: etching another set ofparallel lines with a 45° orientation to the first series of parallellines with the predefined line spacing to create a 0°/45° crosshatchpattern; and moving the at least one laser beam along the 0°/45°crosshatch pattern to create the predefined topographical pattern. 15.The method of claim 12, wherein the at least one laser beam has a widthof 25 micrometers and an energy of 3-175 microJoules.
 16. The method ofclaim 12, wherein: the substrate is composed of a polymer; and thepolymer is selected from a group consisting of polyimide, copoly(imidesiloxane), copoly(imide butadiene), copoly(imidebutadieneacrylonitrile), polycarbonate, poly(arylene ether), fluoropolymer, epoxyresins, and polyphenylene.
 17. The method of claim 12, wherein thepredefined line spacing is 25 micrometers.